Dynamoelectric machines, such as generators, intended for use in aerospace applications present substantial design challenges. For one, they must be lightweight to maximize the useful load carrying capacity of the vehicle in which they are employed.
For another, they must be highly efficient and reliable. The high efficiency requirement is allied to the lightweight requirement in that both objectives are achieved by maximizing the generating capacity per unit of size or weight of the generator.
One means of achieving high efficiency for a given structure is to provide for cooling of the machine. Where heat generated in the electrical generation process is carried off by a coolant, a greater amount of electrical power may be generated on a sustained basis without damaging system components than would be the case without the use of cooling systems.
As a consequence, various cooling schemes have been devised. For example, many large scale generators form windings, particularly stator windings, as conduits and actually flow coolant through the windings themselves. This approach, though successful in large generating stations, cannot be used efficiently in generators intended for aerospace use either because of size limitations or economic limitations or both.
Consequently, many cooling proposals utilize spray cooling of various components of the machines. Not infrequently, the end turns of the stator winding are sprayed with coolant emanating from fixed or movable structure forming part of the generator. This approach, while successful, is not without its difficulties. In particular, the coolant or an air/coolant mixture may tend to find its way into the air gap between the rotor and the stator to cause friction and windage losses, particularly in high speed machines.
A more efficient means of cooling is so-called "back iron" cooling by which a coolant is brought into contact with the stator magnetic material oppositely of the rotor. This approach, however, has not been useable in cooling the stator winding end turns in many instances because of the large volume occupied by such end turns, particularly in a two pole machine, which must be flooded with coolant and which thus may increase the weight of the overall system because of the extremely large volume of coolant required.
Weight concerns are also affected by the physical size of the machine. Thus, while two pole so-called "brushless generators" are generating interest in the aircraft industry because they provide a weight reduction over machines having equal generating power but of the four pole variety, there still remains room for improvement. In particular, in two pole machines, the stator winding when designed according to conventional considerations results in the end turns for such windings having considerable axial length. To accommodate such length, generator housings need be commensurately longer, thus employing a greater amount of housing material, and ultimately increasing the weight of the system over what it might be if the axial length of the end turns could be reduced.
The present invention is directed to overcoming one or more of the above difficulties.